1. Technical Field
The present invention relates generally to one-chip microcomputers and more particularly to a one-chip microcomputer which a commercial product, and has either an input terminal or an output terminal (hereinafter called a selective function or programmable; `I/O terminal`) that allows the selection of an option of functions; namely, the functional selection of, for instance, a CMOS or open drain output state.
2. Background Art
One-chip microcomputers are widely used as control circuits in cameras, home electric appliances, and other kinds of electronic equipment. At the present stage, these are generally 4- or 8-bit one-chip microcomputers. A one-chip microcomputer of this sort is called a microcontroller and different in internal construction from a general purpose microprocessor in that RAM, ROM and the like, each having a fixed capacity, are connected to a bus and also contained in one chip.
The 4- or 8-bit one-chip microcomputer transmits and receives signals via either a input buffer or an output buffer (I/O buffer) to and from an external circuit. Like RAM, ROM and the like, the I/O buffer is also connected to the bus in the chip. These circuits thus connected to the bus are controlled by a processor (or a controller, these are collectively called a CPU in this specification).
In the case of such a one-chip microcomputer having ROM, an application program corresponding to the use of each circuit can be written to the ROM. Consequently, the ROM in the one-chip microcomputer of this sort is loaded with an application program after it has been manufactured as an LSI, depending on its use; that is, it is utilized as a one-chip microcomputer for the intended purpose. The one-chip microcomputer like this is often equipped with selective function that have I/O terminals functional selection of, for instance, a CMOS or open drain output state with respect to one I/O terminal (input or output terminal), the function of the I/O terminal being selectively set at a mask option.
A one-chip microcomputer of the type that has an application program writable to ROM, also allows the application program to be altered, and this arrangement can be utilized for the development of one-chip microcomputers and for various other uses. However, the selective function added to the I/O terminal is unchangeable after a selection is implemented at the mask option.
As a result, the change of the function added to the selective function I/O terminal has been dealt with only through the modification of an external circuit or the provision of an additional external circuit. When the one-chip microcomputer described above is used to develop a system, the fact that the function of the selective function I/O terminal is not simply changeable causes the efficiency of system development to decrease.